Methane in the Amazon
Methane emissions are smaller than those of CO2, and methane abundance in the atmosphere is much smaller. But methane is much more efficient in trapping heat than CO2 is, making it a very important greenhouse gas. Like CO2, methane is emitted by the burning of fossil fuels but it also has many natural sources. They include thawing permafrost and wetlands.
This is where the Amazon rainforest comes into play. The Amazon and its tributaries are bordered by wetlands of continually flooded forest. In addition, river levels swell significantly during the wet season and seasonally flood large areas of otherwise dry forest. Biomass begins to rot in oxygen-depleted water in these wetlands and produces methane. Scientists estimate that up to one-third of all global wetland methane emissions stem from the Amazon rainforest.
Because methane is such an important greenhouse gas, is it crucial that we better understand the natural processes that contribute to methane emissions. We want to figure out what source and sink regions are and how climate change affects them. Yet emissions from tropical wetlands are the single largest source of uncertainty to the global methane budget. Now Santiago Botía and his co-authors analyzed methane in the atmosphere at ATTO. Over the course of five years, they measured methane along with other properties, such as wind speed, wind direction and the stratification of the atmosphere.
Curious methane emission in the dry season
They noticed frequent pulses of methane emissions during the night, but only under certain conditions. The winds always came from the Southeast, the direction where the Uatumã River lies. The atmosphere was also very well stratified above the canopy during those nights and wind speeds comparably low. Surprisingly, these nighttime events mostly occurred in the months of July to September – the dry season in the Amazon. These findings posed a puzzle to our researchers for quite some time.
Eiky Moraes, Cléo Dias-Júnior and their colleagues wanted to find out if the local topography at the ATTO influenced the atmospheric movements. In particular, they were interested in the effect that topography has on the formation of gravity waves. Comparing two simulations, one with and one without topography, revealed some important differences in the dynamics and chemistry of the atmosphere.
Only when the air inside of the forest canopy mixes with the air above can there be exchange. The physical movement of the air, its turbulence, determine how well these two layers of air, the one inside the forest canopy and the one above, mix. Daniela Cava, Luca Mortarini, Cleo Quaresma and their colleagues set out to address some of these questions with two new studies that they conducted at ATTO. They wanted to define the different regimes of atmospheric turbulence or stability (Part 1) and describe the spatial and temporal scales of turbulent structures (Part 2).
Polari Corrêa and his co-authors analyzed the atmospheric dynamics in and above the forest canopy during one particular night at ATTO. Those conditions changed throughout the night. Turbulence was followed by the formation of a gravity wave and a low-level jet. It was likely formed due to the breeze from the Uatumã River and the hilly terrain. The study highlights the complex dynamics and mechanisms in the atmosphere above a dense forest.
Recently we mentioned that drowned trees along the Uatumã River a likely the cause for enhanced methane emissions measured at ATTO. Now Angélica Resende and her co-authors investigated how changes in flooding regimes impact tree mortality in floodplains. They compared two sites in the Amazon basin. Along the Jaú River, the floodplain environment is still largely undisturbed. Along the Uatumã near ATTO, on the other hand, the flooding regime has been altered by the implementation of the Balbina hydroelectric plant further upstream.
Convective storms often occur in the tropics and have the potential to disturb the lower part of the atmosphere. They might even improve the venting of trace gases out of the forest canopy into the atmosphere above. To better understand these processes, Maurício Oliveira and co-authors used the infrastructure at ATTO to study storm outflows during nighttime. They published the results in a new paper in the Open Access Journal Atmospheric Chemistry and Physics.
The Amazon rainforest interacts with the atmosphere by exchanging many substances. Many of these, such as carbon dioxide, methane, ozone, and organic compounds, are produced by the vegetation. They are very influential in both the regional and global climates. Until now, the estimates of their emission and absorption rates are based on classical theories. But those were developed over relatively short vegetation and are valid for the so-called “inertial sublayer.”
Aquino et al. published a new study in Agricultural and Forest Meteorology about the characteristics of turbulence within the forest canopy at two Amazonian sites. They found that the air layer close to ground is largly decouples from the air layer in the upper canopy and above.